This metric is an estimate of an app’s probability of ranking in the top 10 for a keyword. Chance is measured on a scale of 0 to 100, with 100 being the easiest keywords to rank for.
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Basically, we look at the top 10 ranking apps for a keyword and check if most of those apps are strong ones in their category (getting a lot of downloads). If there are very strong apps for a keyword, then it means there's too much competition for that keyword, hence the chance score will be low.